Magnets are indispensable for an eddy current retarder (hereafter, also simply referred to as a retarder) because it generate a magnetic field that provides braking force. Retarders are divided roughly into those using permanent magnets and those electromagnets. In general, the retarder of any type includes a braking member that is fixed to a rotating shaft such as a propeller shaft. During braking, the action of magnetic fields from magnets generates eddy current on the surface of the braking member facing the magnets. This causes braking force in a direction opposite to a rotational direction on the braking member that rotates together with the rotating shaft, which makes the rotation of the rotating shaft slow down. The retarders are categorized into a drum type and a disc type depending on the shape of the braking member and the shape of a magnet retention member that retains the magnets and pairs off with the braking member. There are also various mechanisms for switching between braking and non-braking.
In recent years, along with the prevention of global warming, the improvement in living environment and the reduction in oil dependence have been promoted. For this reason, hybrid electric vehicles (HEVs) and electric vehicles (EVs), which generate motive power for propulsion with an electric motor, have come into widespread use in the areas of not only small vehicles such as cars but also large vehicles such as trucks and buses. In most cases, a large vehicle is equipped with a number of electric components that require electricity. To supply the electricity, a large vehicle is provided with an electric power generator such as an alternator that receives motive power from an engine to generate electricity. However, in large vehicles such as HEVs and EVs, there is a limit to supplying the electricity only by an electric power generator such as an alternator. This is because a propulsion electric motor heavily consumes electricity. For this reason, adding an electricity generating function to a retarder is very useful.
There are prior art retarders with an electricity generating function, as described below.
Japanese Patent Application Publication No. 7-143732 (Patent Literature 1), Japanese Utility Model Laid-Open No. 5-48581 (Patent Literature 2), Japanese Patent Application Publication No. 10-12741 (Patent Literature 3), and Japanese Patent Application Publication No. 2001-54276 (Patent Literature 4) each disclose a drum type, electromagnet-type retarder with an electricity generating function. This retarder includes a cylindrical braking member that is fixed to a rotating shaft of a vehicle, a cylindrical wound coil retention member that is arranged inside and concentric with this braking member, and a permanent magnet retention member that is arranged inside and concentric with this wound coil retention member. The wound coil retention member is of a ferromagnetic material, and is fixed to a non-rotating part of the vehicle. The wound coil retention member retains a plurality of excitation coils (including iron cores) on its outer peripheral surface entirely in its circumferential direction, and retains a plurality of electricity generating coils (including iron cores) on its inner peripheral surface entirely in its circumferential direction. The permanent magnet retention member is integral with the braking member and retains a plurality of permanent magnets on its outer peripheral surface entirely in its circumferential direction.
In the retarders disclosed in Patent Literatures 1 to 4, the excitation coils serve as electromagnets by a current supply, and magnetic circuits develop between the electromagnet and the braking member. Then, with relative rotation between the braking member and the wound coil retention member, magnetic fields from the electromagnets fluctuate. This causes eddy current to be generated on the inner peripheral surface of the braking member, providing braking force. Furthermore, magnetic circuits develop between the electricity generating coils and the permanent magnets. Then, with relative rotation between the permanent magnet retention member and the wound coil retention member, magnetic fields from the permanent magnets penetrating the electricity generating coils fluctuate. This causes induced current to flow in the electricity generating coils, providing electricity.
Japanese Patent Application Publication No. 2011-182574 (Patent Literature 5) discloses a drum type, or a disc type, permanent-magnet type retarder with an electricity generating function. This retarder includes, in the case of drum type one, a cylindrical braking member that is fixed to a rotating shaft of a vehicle, a magnet retention member that is arranged inside and concentric with this braking member, a brake disc that is integral with this magnet retention member, and a brake caliper that is fixed to a non-rotating part of the vehicle with the brake disc interposed therebetween. The magnet retention member is of a ferromagnetic material and rotatably supported by the rotating shaft of the vehicle. The magnet retention member retains a plurality of permanent magnets entirely in its circumferential direction such that the permanent magnets face the inner peripheral surface of the braking member. In the inner peripheral surface of the braking member, a plurality of electricity generating coils (including iron cores) are embedded entirely in its circumferential direction.
In the retarder disclosed in Patent Literature 5, the brake caliper is not operated during non-braking. In this state, with the rotation of the braking member, the action of magnetic fields from the permanent magnets causes the magnet retention member to rotate in synchronization with the braking member. That is, no relative rotational speed difference arises between the braking member and the permanent magnets, and thus the magnetic fields from the permanent magnets do not fluctuate. This prevents eddy current from being generated on the inner peripheral surface of the braking member, generating no braking force. Furthermore, the magnetic fields penetrating the electricity generating coils do not fluctuate either. This prevents induced current from flowing in the electricity generating coils, generating no electricity.
On the other hand, the brake caliper is operated during braking. This causes the rotation of the brake disc to stop, thereby making the magnet retention member come to rest. Then, a relative rotational speed difference arises between the braking member and the permanent magnets, and thus the magnetic fields from the permanent magnets fluctuate. This causes eddy current to be generated on the inner peripheral surface of the braking member, generating braking force. Furthermore, the magnetic fields penetrating the electricity generating coils also fluctuate. This causes induced current to flow in the electricity generating coils, generating electricity.